This application is based on Patent Application No. 2000-335188 filed Nov. 1, 2000 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a bidirectional print apparatus and method for executing bidirectional scanning of print heads that apply ink of a plurality of colors to a print medium, and in particular, to a bidirectional print apparatus and method that can reduce the unevenness of colors which may occur during bidirectional color printing, and prints obtained by this method and apparatus.
2. Description of the Related Art
For print apparatuses, and in particular, those based on an ink jet method, it is important to increase the printing speed during color printing. General methods for increasing the printing speed include an increase in the length of print heads, an increase in the printing frequency of the print heads, and the introduction of bidirectional printing. Compared to unidirectional printing, the bidirectional printing enables required energy to be temporally distributed in order to obtain the same throughput and is thus a cost-efficient means as a total system.
However, with the bidirectional printing method, the landing order of ejected color inks differs between the forward and backward directions of main-scanning depending on the construction of the print apparatus and in particular its print heads. Thus, with this method, unevenly colored portions like bands may be created; this is a problem originating from the principle of this method. Since this problem is caused by the landing order of the ink, more or less a difference in coloring occurs if dots of different colors even partially overlap each other.
If an image is formed by ejecting coloring materials such as pigments or dye inks onto a print medium, the ink in the first printed dot first is deposited on the print medium and then permeates from the front layer to the interior thereof. When ink that forms a subsequent dot is then disposed on the first printed dot so that these dots at least partially overlap each other, a larger amount of ink is placed below an area already dyed with the first ink. Consequently, the first printed dot tends to provide more intense coloring than the subsequent dot. Thus, with a conventional construction in which nozzles for the respective colors are arranged in a main-scanning direction, when bidirectional printing is executed, the landing order of the ejected ink is reversed between the forward and backward scanning. As a result, a difference in coloring occurs to form unevenly colored portions like bands.
This phenomenon occurs not only with ink but also with wax-based coloring materials for forming process colors because of the relationship between the first and second printed dots, in spite of a difference in principle.
Ink jet printers supporting the bidirectional printing are constructed so as to avoid this problem using the following methods:
1) Uneven colors are permitted. Alternatively, only black is printed in both directions.
2) The nozzles for the respective colors are arranged in the sub-scanning direction. That is, what is called a vertical arrangement is used.
3) Forward- and backward-scanning nozzles are provided and nozzles or heads used are switched between the forward and backward scanning so as to use the same landing order for each color (Japanese Patent Application publication No. 3-77066).
4) During forward and backward scanning, rasters are printed so as to be interlaced to complementarily make the color of each printed raster uneven at a high frequency according to a difference in landing order so that the rasters eventually appear even (Japanese Patent Application Publication No. 2-41421 and Japanese Patent Application Laid-open No. 7-112534).
However, the above conventional technique 1) does not provide an essential solution and has the disadvantage of sharply reducing the throughput when a color image is input. With the vertical arrangement in 2) provides the same landing order during both the forward and backward scanning but has the disadvantage of requiring long print heads and being sensitive to a difference in coloring caused by a time difference between landing of each color.
With the method in 3), for example, the formation of forward- and backward-scanning print heads on the same substrate is equivalent to the provision of totally different print heads. Accordingly, this method has the disadvantage of creating band-like unevenly colored portions with a large color difference similar to an inter-head difference. For example, if there is a difference in temperature increase between the print heads associated with a difference in data rate between the forward and backward scanning caused by the interference among the data, a difference in the amount of ink ejected occurs between the print heads to form band-like unevenly colored portions.
This problem is serious in one-pass bidirectional printing, but a similar problem occurs with bidirectional multipass printing owing to a difference in the number of dots printed between the forward and backward printing passes, a difference in the number of dots obtained using a decimation mask for complementing data, or a difference in the number of dots printed associated the yielding of dots to printing of the rasters.
The method in 4) provides uneven colors of a high frequency to make it difficult to visually perceive the unevenness, so that depending on the type of print data, the color difference may be emphasized because of the interference among the data. For example, with an arrangement in which a color difference is created for each raster, if in a halftone portion such as screening, only even rasters are likely appear in a certain area in the forward or backward direction, while only odd rasters are likely appear in a certain area in the backward or forward direction, then a large color difference may occur even if the same color is designated.
It is an object of the present invention to provide a print apparatus and method using an ink jet printing apparatus based on what is called a multipass printing method that forms an image by executing main scanning a plurality of times in the same scanning area using different groups of nozzles, the ink jet printing apparatus being able to reduce the occurrence of uneven colors associated with a scanning direction even when bidirectional color printing is executed. The present invention also provides prints obtained by using this print apparatus and method.
It is another object of the present invention to provide a print apparatus and method that can reduce the occurrence of uneven colors associated with the scanning direction regardless of print data, as well as prints obtained using this print apparatus and method.
To attain this object, the present invention provides a print apparatus comprising print heads having a plurality of printing element rows each having a plurality of printing elements to form a color image by scanning the same scanning area a plurality of times in a forward and a backward directions using different printing element rows of the print heads to apply ink of a plurality of colors to a print medium, wherein the print heads are composed of two sets of printing element array sections each having printing element rows, the sets being sequentially arranged and ejecting different inks, the arrays of the printing element rows of the printing element array sections being set to be symmetrical in a scanning direction, and the print apparatus comprises mask means for complementarily masking predetermined rasters in a raster direction or predetermined columns in a column direction while scanning image data a plurality of times.
Further, the present invention provides a print apparatus comprising print heads having a plurality of printing element rows each having a plurality of printing elements to form a color image by scanning the same scanning area a plurality of times in a forward and a backward directions using different printing element rows of the print heads to apply ink of a plurality of colors to a print medium, wherein the print head are composed of two sets of printing element array sections each having printing element rows, the sets being sequentially arranged and ejecting different inks, the arrays of the printing element rows of the printing element array sections being set to be symmetrical in a scanning direction, and the print apparatus comprises change means for changing an ink application order for at least one of a plurality of secondary color pixel areas arranged in a column direction, compared to an ink application order for the other secondary color pixel areas, to form said secondary color pixel area, and mask means for complementarily masking predetermined rasters in a raster direction or predetermined columns in the column direction while scanning image data a plurality of times.
Furthermore, the present invention provides a print apparatus comprising print heads having a plurality of printing element rows each having a plurality of printing elements to form a color image by scanning the same scanning area a plurality of times in a forward and a backward directions using different printing element rows of the print heads to apply ink of a plurality of colors to a print medium, wherein the print heads are composed of two sets of printing element array sections each having printing element rows, the sets being sequentially arranged and ejecting different inks, the arrays of the printing element rows of the printing element array sections being set to be symmetrical in a scanning direction, and the print apparatus comprises mask means for complementarily masking predetermined rasters in a raster direction or predetermined columns in a column direction while scanning image data a plurality of times.
Moreover, the present invention provides a print method comprising print heads having a plurality of printing element rows each having a plurality of printing elements to form a color image by scanning the same scanning area a plurality of times in a forward and a backward directions using different printing element rows of the print heads to apply ink of a plurality of colors to a print medium, wherein the print heads are composed of two sets of printing element array sections each having printing element rows, the sets being sequentially arranged and ejecting different inks, the arrays of the printing element rows of the printing element array sections being set to be symmetrical in a scanning direction, and the print method comprises a mask step of complementarily masking predetermined rasters in a raster direction or predetermined columns in a column direction while scanning image data a plurality of times.
Furthermore, the present invention provides a print method comprising print heads having a plurality of printing element rows each having a plurality of printing elements to form a color image by scanning the same scanning area a plurality of times in a forward and a backward directions using different printing element rows of the print heads to apply ink of a plurality of colors to a print medium, wherein the print heads are composed of two sets of printing element array sections each having printing element rows, the sets being sequentially arranged and ejecting different inks, the arrays of the printing element rows of the printing element array sections being set to be symmetrical in a scanning direction, and the print method comprises a change step of changing an ink application order for at least one of a plurality of secondary color pixel areas arranged in a column direction, compared to an ink application order for the other secondary color pixel area, to form said secondary color pixel area, and a mask step of complementarily masking predetermined rasters in a raster direction or predetermined columns in a column direction while scanning image data a plurality of times.
With the above arrangement, a plurality of pixel areas arranged in the raster direction and intended for a process color, containing a secondary color, predominantly undergoes the changed application order of the plurality of inks. Accordingly, whether the pixel areas are formed during the forward or backward scanning, the application order does not significantly vary in the raster direction, thereby reducing the occurrence of uneven colors associated with the ink application order.
Further, for multipass printing, by using a mask pattern to apply the change in ink application order to the column direction as well as the raster direction, the occurrence of uneven colors associated with the ink application order can be reduced in both the column and raster directions whether the pixel areas are formed during forward or backward scanning.
Here, the xe2x80x9cprint mediumxe2x80x9d is not limited to paper, used in common print apparatuses, but also means a material that can receive ink, such as a cloth, a plastic film, and a metallic plate.
Further, the xe2x80x9cinkxe2x80x9d should be broadly interpreted in the same manner as the definition of the xe2x80x9cprintxe2x80x9d, and means coloring materials that can be used to form images, patterns, and the like or process the print medium when applied to the surface thereof.
Furthermore, the xe2x80x9cpixel areaxe2x80x9d means the smallest area that represents a primary or secondary color when one or more inks are applied thereto, and includes superpixels and subpixels as well as pixels. Moreover, the number of scanning operations required to complete the pixel area is not limited to one, but a plurality of scanning operations can be performed for this purpose.
Furthermore, the xe2x80x9cprocess colorxe2x80x9d contains a secondary color and is obtained by mixing three or more inks together on the print medium.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.